The present invention relates to a cathode structure of an electron tube, and more particularly to a cathode structure of a magnetron for preventing damage and breakage of a thermal electron cathode of the magnetron.
Since the magnetron generates a high frequency power at a high efficiency it has been widely used in microwave oven and defreezers to heat or defreeze foods or the like. The magnetron basically comprises a ring anode and a thermal electron emitting cathode disposed at the center of the anode. The thermal electron emitting cathode comprises a helical filament which is arranged coaxially with a cylinder axis of the magnetron and fixed by and electrically connected to an upper end shield and a lower end shield which prevent the axial escape of the thermal electrons. The upper end shield is fixed to an upper end of a center lead on the cylinder axis while the lower end shield is fixed to an upper end of a side lead such that the center lead extends through the center of a circular hole in the lower end shield. Terminals are brazed together with stem ceramics to the lower ends of the center lead and the side leads. The filament is fed with electric power through those terminals. Brazed to the stem ceramics are shielding members which constitute portions of a vaccum envelope of the magnetron.
The filament may be made of a thorium tungsten wire which is carbonized to enhance thermal electron emission efficiency. As a result, a carbonized layer is formed on the surface of the wire resulting in the brittleness of the filament. When various external parts are to be mounted on the magnetron having such a filament, the mechanical vibration propagates to the filament through the center lead and the side lead so that the filament may be damaged or broken. When the magnetron is dropped during the handling thereof, the mechanical vibration also propagates to the filament. In order to prevent such vibration, a spacer is placed at a predetermined distance from the lower end shield so that the center lead and the side lead extend therethrough. Since the center lead and the side lead have different lengths, they have different resonance points or resonance frequencies for an external vibration. Accordingly, coupling of the leads by the single spacer serves to prevent the vibration and enhances the resistance to the vibration of the overall cathode. This effect is materially influenced by the position at which the spacer is mounted, and it is desirable from a viewpoint of antivibration to mount the spacer as close to the filament as possible. When the filament is brazed (usually with a high melting point metal such as ruthenium-molybdenum alloy or platinum) to the upper end shield and the lower end shield, a heating filament is arranged around those members. If the spacer is too close to the lower end shield, the spacer may be melted by the high temperature radiation heat. While the spacer is made of ceramic which is known as a relatively high heat resistive material, it must be positioned about 10 mm or more, for example, from the lower end shield to prevent fushion damage. Thus, since the spacer supports the center lead and the side lead at a point distant from the filament, that is, at a point at which the center lead and the side lead vibrate less, the antivibration effect is small. In addition, it has a directivity such that the antivibration effect to the vibration transverse to the direction of the alignment of the center lead and the side lead is small.
On the other hand, the diameters of the center lead and the side lead may be increased to enhance the ridigity thereof in order to enhance the resistance to the vibration of the cathode. However, since those leads are made of expensive molybdenum or tunsten material, it necessarily leads to an increase in the cost and hence it is not desirable.